1. Field of the Invention
The present invention relates to the electrostatic bi-stable display which utilizes Micro-Electro-Mechanical System (MEMS) technology.
2. Background of the Invention
The central idea of the invention is based on the electrostatic movement of the cantilevers from their original position parallel to the substrate plane (off-state) into the position normal to the substrate plane (on-state), thereby modulating light reflection from or light transmission through the display pixels. Electrostatic movement of cantilever pixel design is described in U.S. Pat. Nos. 7,158,278 and 7,362,492, the contents of which are incorporated by reference herein.
FIGS. 1a and 1b, which are comparable to FIG. 2 of the pixel design of the U.S. Pat. No. 7,158,278, illustrate the electrostatic movement of a cantilever assembly 24 as being accomplished by applying a voltage between two electrodes, 22 and 26. The electrode 22 is located on the cantilever (shorter) side while the electrode 26 is placed on the substrate 27. Under the voltage applied, the electrostatic attraction of these electrodes causes the cantilever 24 to rotate into a vertical position around the hinges (not shown) (see FIG. 1b). The cantilever 24 rests on the posts 25, which is typically 3-5 μm (micrometers or microns) tall, to provide a space for the cantilever rotation.
FIG. 2a illustrates a top view and FIG. 2b illustrates a cross-sectional view (through A-A of FIG. 2a) of a conventional cantilever activated display. FIG. 2a is comparable to FIG. 4 of U.S. Pat. No. 7,362,492. The typical hinges, constructed using a MEMS technology on the side of the cantilever, are shown in FIG. 2a, herein, as narrow stripes through which the cantilever is attached to the posts 41. Simultaneously, the stripes 42 provide the inter pixel electrical connection along the vertical direction.
The conventional MEMS technology employs placement of an additional, sacrificial, layer on the substrate to elevate the cantilever plane 3-5 μm above the substrate plane. When this layer is etched, the cantilever rests on the posts 41 and 40 (see FIGS. 2a and 2b).
Typically, in the vertical cantilever position, the inter-electrode capacitance becomes very high. This implies a low voltage is needed to hold the cantilever in the upright or vertical position in comparison with that voltage needed to move the cantilever into the vertical position. Such a bi-stability effect allows simple electrical pixel controlling scheme without AM TFT circuitry.
However, the estimates of the electrostatic force show that the active capacitance between electrodes 22 and 26 of FIG. 1a is too weak and will require unacceptably high voltages for the cantilever elevation. Therefore, the pixel construction must be redesigned to introduce new pixel features and thus overcome this problem.